The present invention relates to the manufacture of substrates. More particularly, the invention provides a reusable donor substrate for separating a thin film of material from. The thin film can be used in the fabrication of a silicon-oninsulator substrate for semiconductor integrated circuits, for example. But it will be recognized that the invention has a wider range of applicability; it can also be applied to other substrates for multi-layered integrated circuit devices, three-dimensional packaging of integrated semiconductor devices, photonic devices, piezoelectronic devices, microelectromechanical systems ("MEMS"), sensors, actuators, solar cells, flat panel displays (e.g., LCD, AMLCD), biological and biomedical devices, and the like.
Wafers for electronic device fabrication are often cut from an ingot, or boule, of material with an abrasive saw. The wafer often serves as both a mechanical substrate and a semiconductor material to form electronic devices in or on. One of the most common examples of this is cutting silicon wafers from a silicon ingot. The wafers are typically polished to a very fine surface finish after removing the mechanical damage left by the abrasive saw. In some processes, devices are fabricated directly in or on the silicon wafer. In other processes, a layer of semiconductor material is grown, for example by epitaxy, on the wafer. An epitaxial layer may provide lower impurity concentrations, or be of a different semiconductor type than the wafer. The devices are formed in what is known as the "active" layer, which is typically only a micron or so thick.
Sawing wafers from an ingot has several disadvantages. First, a significant amount of material may be lost due to the width, or kerf, of the saw blade. Second, the wafers must be cut thick enough to survive a typical circuit fabrication process. As the wafers get larger and larger, the required thickness to maintain sufficient strength to be compatible with given wafer handling methods increases. Third, the polishing process to remove the saw marks takes longer and removes yet more precious material than would be required if an alternative method existed.
The desire to conserve material lost to the sawing and polishing operations increases as the value of an ingot increases. Single-crystal silicon ingots are now being produced with diameters of twelve inches. Each wafer cut and polished from these ingots can cost over a thousand dollars. Ingots of other materials are also being produced. Some of these materials may be difficult to produce as a single crystal, or may require very rare and expensive starting materials, or consume a significant amount of energy to produce. Using such valuable material to provide simple mechanical support for the thin active layer is very undesirable, as is losing material to the sawing and polishing operations.
Several materials are processed by cleaving, rather than sawing. Examples include scribing and breaking a piece of glass, or cleaving a diamond with a chisel and mallet. A crack propagates through the material at the desired location to separate one portion of material from another. Cleaving is especially attractive to separate materials that are difficult to saw, for example, very hard materials. Although the cleaving techniques described above are satisfactory, for the most part, as applied to cutting diamonds or household glass, they have severe limitations in the fabrication of semiconductor substrates. For instance, the above techniques are often "rough" and cannot be used with great precision in fabrication of the thin layers desired for device fabrication, or the like.
From the above, it is seen that a technique for separating a thin film of material from a substrate which is cost effective and efficient is often desirable.